The goals of this work are to understand the oncogenic activity of leukosis viruses in terms of the molecular biology of the viruses. The goals of the current year have been to define murine leukemia viruses structurally and to evaluate the implication of structure on biologic activity. In the past year we have: 1. Determined the sequence of approximately 15% of the ecotropic, N-tropic endogenous AKR virus (Pedersen and Haseltine, 1979). 2. Compared the structure of the leukemogenic Gross A virus with that of the Akv virus. We found the viruses to be similar in structure but to differ in some regions of the genome (Buchhagen, Pedersen, Crowther and Haseltine, 1980). 3. Isolated numerous recombinant DNA clones of proviral sequences related to AKR virus sequences. 4. Obtained some DNA sequence information from a clone of Akv virus provided to us by Dr. Lowy. 5. Determined that a tumor produced upon injection of MCF-247 virus into an AKR mouse produced the MCF-247 virus (Pedersen, Buchhagen, Hays, Chen and Haseltine, 1980). 6. Characterized two new viruses produced by tumors of AKR mice.